Why? All roles can be good at dealing damage, and in fact I think they have to be to justify the way the game is made now. As I see it, it should be like this:
Tanks are good at damaging with abilities that facilitate defense.
Damage heroes are good at damaging with abilities that facilitate offense.
Supports are good at damage with abilities that facilitate supportive gameplay.
With that alone, the damage heroes would be the best at dealing damage, but everyone else can do it since they have to from time to time. Again, that’s the Overwatch everyone asked for.
I don’t think that’s particularly true. In fact, the only reason Tanks do so well is because of their huge health pool. Imagine tanks if they have 250 HP; Genji and Tracer would just out manuver and kill them due to superior mobility and offensive abilities. What could a 250 HP Reinhardt do to Tracer?